Color-separating and -recombining optical system and projection display using the same

ABSTRACT

A color-separating and -recombining optical system includes a cubic- or square column-like first to fourth polarization beam splitters having polarization-splitting planes intersecting each other and wavelength-selective polarizing converters for rotating the plane of polarization of a specific-color light component by 90 degrees. One of the converters is placed at a light-incident side of the first splitter and another is placed at a light-emitting side of the fourth splitter. The first and fourth splitters are provided at light-incident and light-emitting sides, respectively, of the optical system. The first and fourth splitters are arranged diagonally opposing each other. The remaining converters are placed between at least two inner facing planes of the first to fourth splitters. At least the remaining converters and three of the first to fourth splitters are joined to form an optical joint component with a gap between the component and the remaining one splitter.

BACKGROUND OF THE INVENTION

The present invention relates a color-separating and -recombiningoptical system having polarization beam splitters and a projectiondisplay using the optical system.

Color projection displays operate as follows: White light is separatedinto three primary colors R (Red), G (Green) and B (blue). The separatedcolor components are guided to the corresponding spatial lightmodulators (abbreviated to SLM hereinafter) for optical modulation inaccordance with a video signal. The modulated color components arerecombined and projected onto a screen, thus displaying a color imagethereon.

Color projection displays are classified into three types in accordancewith SLMs to be used, such as, a type with SLMs, another with reflectingSLMs, and still another with a DMD (Digital Mirror Device).

Compact projection displays with transparent SLMs and DMDs havingrelatively simple optical architecture are available but have difficultyin resolution.

On the contrary, reflective SLMs exhibit high resolution but pose aproblem in compactness due to complex optical system using this type ofSLMs. Particularly, projection displays equipped with reflective SLMsrequire polarization beam splitters (abbreviated to PBS hereinafter) forsplitting light beams incident to the SLMs and reflected light beamsthat have been modulated by the SLMs. In detail, each reflective SLMrequires two or more of PBSs for high contrast, thus resulting incomplex optical architecture for reflective projection displays.

Colorlink Inc. (US) has proposed a color-separating and -recombiningoptical system having no problem on optical architecture in use ofreflective LSMs, introduced in literature “High Contrast Color SplittingArchitecture Using Color Polarization Filters” by Michael G. Robinsonet., SID 00 DIGEST, 92-95 (2000).

FIG. 1 is a plan view illustrating an optical architecture for aprojection display 300 using reflective SLMs, proposed by Colorlink Inc.

A color-separating and -recombining optical system 290 (enclosed by adot line) has cubic- or square column-like first to fourth PBSs 102,103, 104 and 105 arranged such that polarization-splitting planes 121,131, 141 and 151 intersect each other almost like the character “X”.

First wavelength-selective polarizing converters (G-phase plates) 106and 107 are provided on the light-incident plane side of the first PBS102 (the left side of the PBS 102 in FIG. 1) and light-emitting planeside of the fourth PBS 105 (the right side of the PBS 105 in FIG. 1),respectively, for rotating the plane of polarization of aG-linearly-polarized light by 90 degrees.

Second wavelength-selective polarizing converters (R-phase plates) 108and 109 are provided between the first and the third PBSs 102 and 104,and the third and the fourth PBSs 104 and 105, respectively, forrotating the plane of polarization of a R-linearly-polarized light by 90degrees.

Linearly-polarized light is classified into S-polarized light andP-polarized light. A polarized light is decided as S- or P-polarizedlight in accordance with relativity between its plane of polarizationand a polarization-splitting plane of a PBS to which it is incident. Inother words, a polarized light is called S-polarized light when itsplane of polarization is orthogonal to an incident plane against apolarization-splitting plane of a PBS, whereas it is called P-polarizedlight when its plane of polarization is horizontal to the incidentplane.

The projection display 300 has a relatively simple optical architecturefor high contrast even though it requires three PBSs for each reflectiveSLM.

Nonetheless, this projection display has a problem of low contrast atthe corners of a black image screen due to birefringence caused by atransparent material for the PBSS due to wrong choice for thetransparent material in the projection display 300 when a high-intensitydischarge lamp of 100 W or more is used.

Japanese-Unexamined Patent Publication No. 9-54213 discloses that atransparent material of 1.5×10⁻⁸ cm²/N or less as the absolute value ofopto-elastic constant is suitable for such PBSs.

It is disclosed that a transparent material of low opto-elastic constantis suitable at least for a main (reflective) PBS that splits incidentlight and light reflected therefrom after modulation.

The inventors of the present invention have, however, found that theproblem discussed above cannot be solved by employing such transparentmaterial of low opto-elastic constant when it is used only for the mainPBSs (the second and the third PBSs 103 and 104) for the projectiondisplay 300 equipped with the color-separating and -recombining opticalsystem 290.

The above problem could be solved by employing such transparent materialof low opto-elastic constant when it is used for all of the four PBSs,which, however, results in high cost for the color-separating and-recombining optical system.

Such transparent material of low opto-elastic constant is generallyseveral times or several ten times more expensive than usual opticalglass such as BK7 because it contains much lead and hence too weak andsoft for machining.

Moreover, the color-separating and -recombining optical system 290,offered by Colorlink Inc., has revealed low reliability because alloptical elements of the optical system 290 joined by an adhesive werepeeled off from each other at a thermal-cyclic reliability test.

The following is a possible reason for low reliability:

As already described, the color-separating and recombining opticalsystem 290 has four PBSs 102, 103, 104 and 105 arranged such that theirpolarization-splitting planes 121, 131, 141 and 151 intersect each otheralmost like the character “X”.

In the reliability test, the optical elements were subjected to thermalexpansion and contraction while the optical system 290 were being heatedand cooled cyclically. Stress was then generated from the center of theintersection of the four PBSs in the direction of circumference due tothermal expansion and contraction. The circumferential stress couldcause outward shear stress in heating whereas tensile stress in coolingat each joint section of the optical elements, thus resulting ispeeling-off for the optical elements from each other.

The character-“X”-like arrangements of the PBSs 102, 103, 104 and 105also poses the following problem:

As illustrated in FIG. 2, some components of light incident to the firstPBS (light-incident-side PBS) 102 are further incident to the fourth PBS(light-emitting-side PBS) 105. The unnecessary light components L areprojected onto a screen (not shown) via a projection lens 191, togenerate bright portions on the screen, thus resulting in low qualityfor images displayed thereon. The image quality will be further loweredwhen the four PBSs 102, 103, 104 and 105 are bonded each other by ajoint material 110 such as a transparent adhesive.

When the color-separating and -recombining optical system 290 hasintegrators on reflective SLMs 161, 162 and 163 at the light-sourceside, an integrator-segment image is displayed on screen while light isilluminating these SLMs. However, light components spread over theperiphery of each reflective SLM could also become the unnecessary lightcomponents L projected onto the screen.

In addition, light components reflected from the reflective SLMs 161,162 and 163 could be reflected again at a first polarizing plate 181 andbecome the unnecessary light components L projected onto the screen.

SUMMARY OF THE INVENTION

Under consideration of the problems discussed above, a purpose of thepresent invention is to provide a low-cost but highly-reliablecolor-separating and -recombining optical system having polarizationbeam splitters unsusceptible to birefringence, suitable for use inreflective projection display.

Another purpose of the present invention is to provide ahigh-image-quality color-separating and -recombining optical system witha function of preventing unnecessary light components being displayed ona screen and a projection display using this optical system.

The present invention provides a color-separating and -recombiningoptical system comprising: cubic- or square column-like first to fourthpolarization beam splitters having polarization-splitting planesintersecting each other like a character-“X”; and wavelength-selectivepolarizing converters each for rotating the plane of polarization of aspecific-color light component by 90 degrees, one of the convertersbeing placed at a light-incident side of the first splitter, another ofthe converters being placed at a light-emitting side of the fourthsplitter, the first and the fourth splitters being provided at alight-incident side and a light-emitting side, respectively, of theoptical system, the first and the fourth splitters being arranged asdiagonally opposing each other, and the remaining converters beingplaced between at least two inner facing planes of the first to thefourth splitters, wherein at least the remaining converters and three ofthe first to the fourth splitters are joined each other to form anoptical joint component with a gap between the remaining one splitter.

Moreover, the present invention provides a color-separating and-recombining optical system comprising: cubic- or square column-likefirst to fourth polarization beam spltters having polarization-splittingplanes intersecting each other like a character-“X”; andwavelength-selective polarizing converters each for rotating the planeof polarization of a specific-color light component by 90 degrees, oneof the converters being placed at a light-incident side of the firstsplitter, another of the converters being placed at a light-emittingside of the fourth splitter, the first and the fourth splitters beingprovided at a light-incident side and a light-emitting side,respectively, of the optical system, the first and the fourth splittersbeing arranged as diagonally opposing each other, and the remainingconverters being placed between at least two inner facing planes of thefirst to the fourth splitters, wherein opto-elastic constants for thefirst to the fourth splitters have a relationship Ki<Km and Ko, Ki andKm<Ko or Ki<Km<Ko in which Ki, Km and Ko denote the opto-elasticconstants for the first splitter, the second and the third splitters andthe fourth splitter, respectively.

Moreover, the present invention provides a colorseparating and-recombining optical system comprising: cubic- or square column-likefirst to fourth polarization beam spltters having polarization-splittingplanes intersecting each other like a character-“X”;wavelength-selective polarizing converters each for rotating the planeof polarization of a specific-color light component by 90 degrees, oneof the converters being placed at a light-incident side of the firstsplitter, another of the converters being placed at a light-emittingside of the fourth splitter, the first and the fourth splitters beingprovided at a light-incident side and a light-emitting side,respectively, of the optical system, the first and the fourth splittersbeing arranged as diagonally opposing each other, and the remainingconverters being placed between at least two inner facing planes of thefirst to the fourth splitters; and a light blockage provided at anintersection of the polarization-splitting planes and surrounded by thefirst to the fourth splitters, the light blockage preventing lightleakage from the first to the fourth splitters.

Furthermore, the present invention provides a color-separating and-recombining optical system comprising: cubic-or square column-likefirst to fourth polarization beam splitters havingpolarization-splitting planes intersecting each other like acharacter-“X”; wavelength-selective polarizing converters each forrotating the plane of polarization of a specific-color light componentby 90 degrees, one of the converters being placed at a light-incidentside of the first splitter, another of the converters being placed at alight-emitting side of the fourth splitter, the first and the fourthsplitters being provided at a light-incident side and a light-emittingside, respectively, of the optical system, the first and the fourthsplitters being arranged as diagonally opposing each other, and theremaining converters being placed between at least two inner facingplanes of the first to the fourth splitters; and first light blockagesprovided at a first corner of the cubic- or square column-like firstsplitter and a second corner of the cubic- or square column-like fourthsplitter, edges of the first and the second corners diagonally opposingeach other being cut off to be flat to face each other, the first lightblockages preventing light leakage from the first to the fourthsplitters.

Moreover, the present invention provides a projection displaycomprising: a light source for emitting unlinearly-polarized light; afirst polarizer to allow only a first specific-linearly-polarized lightcomponent of the unlinearly-polarized light to pass therethrough; acolor-separating and -recombining optical system including cubic- orsquare column-like first to fourth polarization beam splitters havingpolarization-splitting planes intersecting each other like acharacter-“X ”, the first splitter being provided as facing the firstpolarizer, and wavelength-selective polarizing converters each forrotating the plane of polarization of a specific-color light componentby 90 degrees, one of the converters being placed at a light-incidentside of the first splitter, another of the converters being placed at alight-emitting side of the fourth splitter, the first and the fourthsplitters being provided at a light-incident side and a light-emittingside, respectively, of the optical system, the first and the fourthsplitters being arranged as diagonally opposing each other, and theremaining converters being placed between at least two inner facingplanes of the first to the fourth splitters, wherein at least theremaining converters and three of the first to the fourth splitters arejoined each other to form an optical joint component with a gap betweenthe remaining one splitter; reflective spatial light modulators forlight modulation in accordance with a video signal, provided outside theoptical system, as facing each light-passing plane of the second and thethird splitters, a second polarizer provided as facing a light-emittingside plane of the fourth splitter, to allow only a secondspecific-linearly-polarized light component emitted from thelight-emitting side plane of the fourth splitter to pass therethrough;and a projection lens provided as facing the second polarizer, toreceive the second specific-linearly-polarized light component for imageprojection.

Furthermore, the present invention provides a projection displaycomprising: a light source for emitting unlinearly-polarized light; afirst polarizer to allow only a first specific-linearly-polarized lightcomponent of the unlinearly-polarized light to pass therethrough; acolor-separating and -recombining optical system including cubic- orsquare column-like first to fourth polarization beam splitters havingpolarization-splitting planes intersecting each other like acharacter-“X”, the first splitter being provided as facing the firstpolarizer, and wavelength-selective polarizing converters each forrotating the plane of polarization of a specific-color light componentby 90 degrees, one of the converters being placed at a light-incidentside of the first splitter, another of the converters being placed at alight-emitting side of the fourth splitter, the first and the fourthsplitters being provided at a light-incident side and a light-emittingside, respectively, of the optical system, the first and the fourthsplitters being arranged as diagonally opposing each other, and theremaining converters being placed between at least two inner facingplanes of the first to the fourth splitters, wherein opto-elasticconstants for the first to the fourth splitters have a relationshipKi<Km and Ko, Ki and Km<Ko or Ki<Km<Ko in which Ki, Km and Ko denote theopto-elastic constants for the first splitter, the second and the thirdsplitters and the fourth splitter, respectively; reflective spatiallight modulators for light modulation in accordance with a video signal,provided outside the optical system, as facing each light-passing planeof the second and the third splitters, a second polarizer provided asfacing a light-emitting side plane of the fourth splitter, to allow onlya second specific-linearly-polarized light component emitted from thelight-emitting side plane of the fourth splitter to pass therethrough;and a projection lens provided as facing the second polarizer, toreceive the second specific-linearly-polarized light component for imageprojection.

Furthermore, the present invention provides a projection displaycomprising: a light source for emitting unlinearly-polarized light; afirst polarizer to allow only a first specific-linearly-polarized lightcomponent of the unlinearly-polarized light to pass therethrough; acolor-separating and -recombining optical system including cubic- orsquare column-like first to fourth polarization beam splitters havingpolarization-splitting planes intersecting each other like acharacter-“X”, the first splitter being provided as facing the firstpolarizer, wavelength-selective polarizing converters each for rotatingthe plane of polarization of a specific-color light component by 90degrees, one of the converters being placed at a light-incident side ofthe first splitter, another of the converters being placed at alight-emitting side of the fourth splitter, the first and the fourthsplitters being provided at a light-incident side and a light-emittingside, respectively, of the optical system, the first and the splittersbeing arranged as diagonally opposing each other, and the remainingconverters being placed between at least two inner facing planes of thefirst to the fourth splitters, and a light blockage provided at anintersection of the polarization-splitting planes and surrounded by thefirst to the fourth splitters, the light blockage preventing lightleakage from the first to the fourth splitters; reflective spatial lightmodulators for light modulation in accordance with a video signal,provided outside the optical system, as facing each light-passing planeof the second and the third splitters, a second polarizer provided asfacing a light-emitting side plane of the fourth splitter, to allow onlya second specific-linearly-polarized light component emitted from thelight-emitting side plane of the fourth splitter to pass therethrough;and a projection lens provided as facing the second polarizer, toreceive the second specific-linearly-polarized light component for imageprojection.

Furthermore, the present invention provides a projection displaycomprising: a light source for emitting unlinearly-polarized light; afirst polarizer to allow only a first specific-linearly-polarized lightcomponent of the unlinearly-polarized light to pass therethrough; acolor-separating and -recombining optical system including cubic- orsquare column-like first to fourth polarization beam splitters havingpolarization-splitting planes intersecting each other like acharacter-“X”, the first splitter being provided as facing the firstpolarizer, wavelength-selective polarizing converters each for rotatingthe plane of polarization of a specific-color light component by 90degrees, one of the converters being placed at a light-incident side ofthe first splitter, another of the converters being placed at alight-emitting side of the fourth splitter, the first and the fourthsplitters being provided at a light-incident side and a light-emittingside, respectively, of the optical system, the first and the splittersbeing arranged as diagonally opposing each other, and the remainingconverters being placed between at least two inner facing planes of thefirst to the fourth splitters, and light blockages provided at a firstcorner of the cubic- or square column-like first splitter and a secondcorner of the cubic- or square column-like fourth splitter, edges of thefirst and the second corners diagonally opposing each other being cutoff to be flat to face each other, the light blockages preventing lightleakage from the first to the fourth splitters; reflective spatial lightmodulators for light modulation in accordance with a video signal,provided outside the optical system, as facing each light-passing planeof the second and the third splitters, a second polarizer provided asfacing a light-emitting side plane of the fourth splitter, to allow onlya second specific-linearly-polarized light component emitted from thelight-emitting side plane of the fourth splitter to pass therethrough;and a projection lens provided as facing the second polarizer, toreceive the second specific-linearly-polarized light component for imageprojection.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view illustrating a known optical architecture for aprojection display having reflective spatial light modulators (SLMs);

FIG. 2 is another plan view illustrating the known optical architecturefor a projection display having reflective SLMs;

FIG. 3 is a plan view illustrating the first embodiment of acolor-separating and -recombining (CSR) optical system according to thepresent invention;

FIG. 4 illustrates effects of birefringence to images on screen;

FIG. 5 illustrates black image screens for a normal black image and abirefringence-affected black image;

FIG. 6 illustrates a surface temperature measured on each polarizationbeam splitter (PBS) in the first embodiment;

FIG. 7 is a plan view illustrating the second embodiment of a CSRoptical system according to the present invention;

FIG. 8 is a plan view illustrating the third embodiment of a CSR opticalsystem according to the present invention;

FIG. 9 is a plan view illustrating the fourth embodiment of a CSRoptical system according to the present invention;

FIG. 10 is a plan view illustrating a sample CSR optical systemrepresentative of several samples used in birefringence experiments;

FIG. 11 shows results of the birefringence experiments for 15 sample CSRoptical systems made of several glass types different in opto-elasticconstant for polarization beam splitters;

FIG. 12 indicates evaluation criteria for the birefringence experimentsand also illustrating birefringence-evaluation models;

FIG. 13 is a plan view illustrating the fifth embodiment of a CSRoptical system according to the present invention;

FIG. 14 is a plan view illustrating a projection display as the sixthembodiment of the present invention;

FIG. 15 is a plan view illustrating a projection display as the seventhembodiment of the present invention;

FIG. 16 is a plan view illustrating a projection display as the eighthembodiment of the present invention; and

FIG. 17 is a plan view illustrating a projection display as the ninthembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments according to the present invention will bedisclosed with reference to the attached drawings.

[First Embodiment]

FIG. 3 is a plan view illustrating the first embodiment of acolor-separating and -recombining optical system according to thepresent invention.

A color-separating and -recombining optical system (abbreviated to CSRoptical system hereinafter) 1 has cubic- or square column-like first tofourth polarization beam splitters (abbreviated to PBS hereinafter) 2,3, 4 and 5 arranged such that polarization-splitting planes 21, 31, 41and 51 intersect each other almost like the character “X”.

The first PBS 2 is a light-incident-side PBS whereas the opposing fourthPBS 5 is a light-emitting-side PBS. It is a matter of choice as to whichbeam splitter is set as the light-incident-side PBS. One requirement isthat any PBS that opposes the light-incident-side PBS be set as thelight-emitting-side PBS.

A first wavelength-selective polarizing converter (G-phase plate) 6 isprovided in front of a light-passing plane 2 a (the light-incident planeside) selected from among light-passing planes 2 a, 2 b, 2 c and 2 d ofthe first PBS (light-incident-side PBS) 2. Another firstwavelength-selective polarizing converter (G-phase plate) 7 is providedbehind a light-passing plane 5 c (the light-emitting plane side)selected from among light-passing planes 5 a, 5 b, 5 c and 5 d of thefourth PBS (light-emitting-side PBS) 5. Each wavelength-selectivepolarizing converter rotates the plane of polarization ofG-linearly-polarized light by 90 degrees.

Second wavelength-selective polarizing converters (R-phase plates) 8 and9 are provided between the first and the third PBSs 2 and 4, and thethird and the fourth PBSs 4 and 5, respectively, for rotating the planeof polarization of a R-linearly-polarized light by 90 degrees.

Except the G-phase plate 6 and also the first PBS 2, the G-phase plate7, the R-phase plates 8 and 9 and the second to fourth PBSs 3 to 5 arejoined each other by a joint material 10 such as an adhesive to form anoptical joint component 11.

The G-phase plate 6 and the first PBS 2 may be attached to each other orseparated as shown in FIG. 3 with a gap. It is also preferable that theR-phase plate 8 is attached to the first PBS 2 whereas it is separatedfrom the third PBS 4.

A spatial light modulator (SLM) 61 for G-light component is placed atthe side of a light-passing plane 3 c of the second PBS 3. Placed asfacing a light-passing plane 4 b of the third PBS 4 is an SLM 62 forR-light component. Moreover, an SLM 63 for B-light component is placedat the side of a light-passing plane 4 a of the third PBS 4.

The CSR optical system 1 as the first embodiment of the presentinvention separates R-, G- and B-light components from white light andrecombines them as follows:

The S-polarized light of the white light is incident to the G-phaseplate 6 for rotating the plane of polarization of G-light component onlyby 90 degrees. Among the light components of the white light passedthrough the G-phase plate 6, the G-light component (solid line in FIG.3) only is converted from the S-polarized light to P-polarized light dueto polarization conversion. R-light component (one-dot dashed line inFIG. 3) and B-light component (two-dot dashed line in FIG. 3) remainunchanged as the S-polarized light.

The transition of light path and plane of polarization will be explainedfor each of R-, G- and B-light components.

Explained first is the G-light component that has passed through theG-phase plate 6.

The G-light component (solid line) has been converted into theP-polarized light as described above. The G-light component passesthrough the polarization-splitting planes 21 and 31 of the first and thesecond PBSs 2 and 3, respectively. The G-light component is emitted froma light-passing plane 3 c and incident to the reflective SLM 61 forlight modulation in accordance with a video signal corresponding to theG-light component. The modulated G-light component returns to thepolarization-splitting plane 31 of the second PBS 3.

The S-polarized light generated for the G-light component due to lightmodulation is reflected at the polarization-splitting plane 31 towardsthe polarization-splitting plane 51 of the fourth PBS 4. The G-lightcomponent (S-polarized light) is reflected at the polarization-splittingplane 51 and emitted from a light-passing plane 5 c of the fourth PBS 4,and thus being incident to the G-phase plate 7 for rotating the plane ofpolarization of the G-light component only by 90 degrees. TheS-polarized light of the G-light component is thus converted intoS-polarized light and emitted from the G-phase plate 7.

Explained next is the R-light component (one-dot dashed line).

The R-light component (S-polarized light) that has passed through theG-phase plate 6 is reflected at the polarization-splitting plane 21 ofthe first PBS 2 and incident to the R-phase plate 8 for rotating theplane of polarization of R-light component only by 90 degrees. TheS-polarized light for the R-light component is thus converted intoP-polarized light and emitted from the R-phase plate 8.

The R-light component (P-polarized light) then passes through thepolarization-splitting plane 41 and emitted from a light-passing plane 4b of the third PBS 4. The emitted R-light component is incident to thereflective SLM 62 for light modulation in accordance with a video signalcorresponding to the R-light component.

The S-polarized light generated for the R-light component due to lightmodulation is reflected at the polarization-splitting plane 41 of thethird PBS 3 towards the R-phase plate 9 for rotating the plane ofpolarization of R-light component only by 90 degrees. The S-polarizedlight of the R-light component is thus converted into P-polarized lightand incident to the fourth PBS 5.

The R-light component (P-polarized light) passes through thepolarization-splitting plane 51 and is emitted from the light-passingplane 5 c of the fourth PBS 4. The emitted R-light component is incidentto the G-phase plate 7. The R-light component (P-polarized light)incident to the G-phase plate 7 is not subjected to rotation ofpolarization plane as described above, and hence emitted therefrom asthe P-polarized light.

Explained further is the B-light component (two-dot dashed line).

The B-light component (S-polarized light) that has passed through theG-phase plate 6 is reflected at the polarization-splitting plane 21 ofthe first PBS 2 and incident to the R-phase plate 8. The B-lightcomponent (S-polarized light) incident to the R-phase plate 8 is notsubjected to rotation of polarization plane as described above, andhence emitted therefrom as the S-polarized light and incident to thethird PBS 4.

The B-light component (S-polarized light) is reflected at thepolarization-splitting plane 41 of the third PBS 4 and emitted from alight-passing plane 4 a, and thus incident to the reflective SLM 63 forlight modulation in accordance with a video signal corresponding to theB-light component.

The P-polarized light generated for the B-light component due to lightmodulation passes through the polarization-splitting plane 41 of thethird PBS 4 and incident to the R-phase plate 9. The B-light component(P-polarized light) incident to the G-phase plate 9 is not subjected torotation of polarization plane as described above, and hence emittedtherefrom as the P-polarized light.

The B-light component (P-polarized light) passes through thepolarization splitting plane 51 of the fourth PBS 5. The B-lightcomponent (P-polarized light) is emitted from the light-passing plane 5c of the fourth PBS 4, and thus incident to the G-phase plate 9.

The B-light component (P-polarized light) incident to the G-phase plate9 is not subjected to rotation of polarization plane as described above,and hence emitted therefrom as the P-polarized light.

As disclosed above, the white light incident to the CSR optical system 1via the G-phase plate 6 is separated into the R-, G- and B-lightcomponents and recombined as all of the three light components areconverted into P-polarized lights at their polarization plane, andemitted from the G-phase plate 7.

When usual optical glass such as BK7 is used for the CSR optical system1, it will cause low contrast at the corners of a black image screen,which exhibit high intensity at the peripheral sections of the blackimage due to birefringence occurring on the optical elements.

Birefringence will occur due to thermal stress generated while lightbeams are passing through the optical elements or external mechanicalstress. For example, a big temperature difference between the center ofa square column-like optical element and the corners of the squarecolumn could cause thermal stress concentrated on the square corners,thus resulting in birefringence at the corners.

When a linearly-polarized light passes through an optical element inwhich birefringence is occurring, the plane of polarization of thelinearly-polarized light is rotated, which then affects images onscreen.

This phenomenon is explained in detail with reference to FIG. 4.Illustrated in FIG. 4 is a projector having one PBS 22 and onereflective SLM 24 and a projection lens 25.

The above-discussed phenomenon in which a black image screen suffers lowcontrast at its corners is the most significant problem due tobirefringence affecting images on screen.

A black image is displayed when light incident to the SLM 24 through aspecific light path returns to the path inversely with no lightmodulation. In detail, in FIG. 4, S-polarized light components (solidline) incident to the PBS 22 are reflected at a polarization-splittingplane 23 and incident to the reflective SLM24. The reflected lightcomponents are the S-polarized light unchanged with no modulation whiledisplaying a black image. The S-polarized light components reflected atthe reflective SLM 24 are further reflected at thepolarization-splitting plane 23 again and return to the light pathinversely, thus a black image being displayed on screen via theprojection lens 25.

When birefringence occurs on the transparent material of the PBS 22, asindicated by oblique lines in FIG. 4, S-polarized light componentspassing therethrough are subjected to rotation of polarization plane,thus generating P-polarized light components, as indicated by dot linesin FIG. 4. The P-polarized light components pass through thepolarization splitting plane 23 and generate bright portions at thecorners of a black image screen via the projection lens 25. This is thephenomenon discussed so far caused by birefringence.

FIG. 5 illustrates black image screens in which FIG. 5(a) is anillustration of a normal black image whereas FIG. 5(b) bright portionsappeared at the corners of a black image due to birefringence.

One of the causes of birefringence is thermal stress as alreadydiscussed. The surface temperature on the first to the fourth PBSs 2 to5 of the CSR optical system 1 (FIG. 3) were measured for thermal stress,as shown in FIG. 6.

The light source used in this temperature measurement was an extra-highpressure mercury lamp. The surface temperature on each PBS was measuredwhen one hour elapsed after the light source had been turned on at roomtemperature.

The results were as follows:

The surface temperature on the first PBS 2 (light-incident-side PBS) wasthe highest, particularly, 39° C. the highest on a triangular prism 2A.The second highest was 37.3° C. on a triangular prism 4A of the thirdPBS 4. The lowest was in the range from 29° C. to 30° C. on the fourthPBS 5. It was found that the higher the intensity for the light source,the larger the temperature difference.

A possible reason for the highest surface temperature on the triangularprism 2A of the first PBS 2 is that all of the R-, G- and B-lightcomponents are incident to the prism 2A and emitted therefrom, andincident thereto again after reflected at the other optical elements.

A possible reason for the second highest surface temperature on thetriangular prism 4A of the third PBS 4 is that the R- and B-lightcomponents are incident to the prism 4A and emitted therefrom, andincident thereto again after reflected at the other optical elements.

Moreover, a possible reason for the lowest surface temperature on thefourth PBS 5 is that substantially no light components are incidentthereto.

The measurement revealed that the first PBS 2 suffered the highestsurface temperature compared to any of the other PBSs.

The following is a possible explanation for the known CSR optical system290 (FIG. 1) having all optical elements joined by an adhesive. Thesurface temperature difference among the optical elements causesdifference in thermal expansion among the optical elements, whichresults in stress and hence birefringence occurred to each opticalelement, especially, the first PBS 102 that suffers the largesttemperature increase while in use.

On the contrary, the CSR optical system 1 (FIG. 3) as the firstembodiment of the present invention suffered fur less generation ofbirefringence. A possible reason lies in the optical arrangements inthat the first PBS 2 is separated, via gap, from the optical jointcomponent 11 formed of the G-phase plate 7, the R-phase plates 8 and 9and the second to fourth PBSs 3 to 5 bonded each other by the jointmaterial 10 such as an adhesive. The arrangements could serve torestrict stress to the first PBS 2, which may otherwise occur due to thetemperature difference between the PBS 2 and the other optical elements.

The thermal-cyclic reliability test was also conducted to the CSRoptical system 1. The result was no peeling-off for the optical elementsat the joint sections. A possible reason is as follows: Stress wasgenerated from the center of the intersection of the fourpolarization-splitting planes 21, 31, 41 and 51 in the direction ofcircumference due to thermal expansion and contraction during thethermal-cyclic reliability test. The stress was, however, released atthe first PBS 2 separated from the optical joint component 11, resultingin no outward shear and tensile stress which were generated for theknown optical system 290.

[Second Embodiment]

FIG. 7 is a plan view illustrating the second embodiment of a CSRoptical system according to the present invention.

Elements in this embodiment that are the same as or analogous to theelements in the first embodiment (FIG. 3) are referenced by the samereference numbers and will not be explained in detail.

In a CSR optical system 50, a first PBS (the light-incident-side PBS) 2and a G-phase plate 6 provided at the light-incident side are attachedto an optical joint component 11 (in which second to fourth PBSs 3 to 5,a G-phase plate 7 and R-phase plates 8 and 9 are joined each other) by abuffer material 12 such as a transparent adhesive or coupling oil.

The first PBS 2 and the G-phase plate 6 may be joined by a jointmaterial 10 or the buffer material 12. The third PBS 4 and the R-phaseplate 8 may be separated from each other via gap instead of being joinedby the joint material 10.

The second embodiment is equivalent to the first embodiment inoperation, and hence not disclosed for brevity.

According to the second embodiment, like the first embodiment, the firstPBS (light-incident-side PBS) 2 is separated from the optical jointcomponent 11 via the buffer material 12. This optical arrangement servesto restrict thermal stress to the first PBS 2, which may otherwise occurdue to temperature difference between the first PBS 2 and the opticaljoint component 11, even when the PBS2 is heated to a temperature higherthan the component 11 while light is passing the PBS 2, thus causingless birefringence.

The thermal-cyclic reliability test also showed that the stressgenerated by thermal expansion and contraction of each optical elementwas released at the first PBS 2, resulting no peeling-off for theoptical elements from each other at the joint sections.

[Third Embodiment]

FIG. 8 is a plan view illustrating the third embodiment of a CSR opticalsystem according to the present invention.

Elements in this embodiment that are the same as or analogous to theelements in the first embodiment (FIG. 3) are referenced by the samereference numbers and will not be explained in detail.

In a CSR optical system 80, all of first to fourth PBSs 2, 3, 4 and 5,G-phase plates 6 and 7, and also R-phase plates 8 and 9 are joined eachother by a joint material 10, with a slit 13 (gap) between the first PBS2 and the R-phase plate 8. The G-phase plate 6 and the first PBS 2 maybe separated from each other or joined by a transparent buffer material.The slit 13 may also be filled with a transparent buffer material.

The third embodiment is equivalent to the first embodiment in operation,and hence not disclosed for brevity.

According to the third embodiment, the slit 13 provided between thefirst and the third PBSs 2 and 4 which tend to be heated to a hightemperature while light is passing serves to release the stress due todifference in thermal expansion for the optical elements, thus resultingin less generation of birefringence.

Moreover, the slit 13 serves to release the stress generated from thecenter of the intersection of polarization-splitting planes 21, 31, 41and 51 of the four PBSs in the direction of circumference due to thermalexpansion and contraction, resulting in no peeling-off for the opticalelements from each other at the joint sections.

[Fourth Embodiment]

FIG. 9 is a plan view illustrating the fourth embodiment of a CSRoptical system according to the present invention.

Elements in this embodiment that are the same as or analogous to theelements in the first embodiment (FIG. 3) are referenced by the samereference numbers and will not be explained in detail.

A CSR optical system 90 is equivalent to the optical system 80 in FIG. 8except that a slit (gap) 13 is provided between first and second PBSs 2and 3, as shown in FIG. 9. The slit 13 may be filled with a transparentbuffer material.

The fourth embodiment is equivalent to the first embodiment inoperation, and hence not disclosed for brevity.

According to the fourth embodiment, the slit 13 provided between thefirst PBS 2 which tends to be heated to a high temperature while lightis passing and the second PBS 3 (instead of the third PBS 4 in FIG. 8)serves to release the stress due to difference in thermal expansion forthe optical elements, thus resulting in less generation ofbirefringence.

This slit 13 also serves to release the stress generated from the centerof the intersection of polarization-splitting planes 21, 31, 41 and 51of the four PBSs in the direction of circumference due to thermalexpansion and contraction, resulting in no peeling-off for the opticalelements from each other at the joint sections.

[Discussion on Opto-Elastic Constant of PBS-Transparent material]

It is a known fact that a polarization beam splitter suffersbirefringence in accordance with an opto-elastic constant of atransparent material for the beam splitter. It is also known thatglasses as an optical material of low opto-elastic constant areexpensive.

Under consideration of these facts, CSR optical systems should bedesigned in accordance with projection and economical efficiency.

Birefringence to CSR optical systems will be discussed throughexperiments on opto-elastic constant of glasses as an optical materialfor PBSs.

FIG. 10 is a plan view illustrating a sample CSR optical system 100representative of several samples used in the experiments.

Elements of the CSR optical systems 100 that are the same as oranalogous to elements in the first embodiment (FIG. 3) are referenced bythe same reference numbers and will not be explained in detail.

Several sample CSR optical system 100 were made by joining all opticalelements (first to fourth PBSs 2 to 5, G-phase plates 6 and 7, andR-phase plates 8 and 9) with a joint material 10 such as a transparentadhesive.

Experimental results are shown in FIG. 11. Birefringence was evaluatedon 15 sample CSR optical systems 100 made of several glass typesdifferent in opto-elastic constant for the first to fourth PBSs 2 to 5.

FIG. 12 indicates the evaluation criteria as follows:

X useless due to much birefringence

Δ useful only for low-quality projection display

∘ birefringence noticed by close observation

⊚ no birefringence observed

FIG. 12 also illustrates four birefringence-evaluation models, thoughbirefringence will be observed in more different ways, which depend onevaluation conditions.

The experiments were conducted using a relatively low intensity 100W-UHP lamp, a intermediate-intensity 150 W-UHP lamp (Philips Co.-madeextra-pressure mercury lamps) and also a high intensity 200 W-UHL lamp(Ushio Co.-made extra-pressure mercury lamp).

Discussed first is birefringence against the 100 W-light source.

The sample 1 was made of BK7 (SCHOTT made) having opto-elastic constantof 2.77×10⁻⁸ cm²/N for the first to the fourth PBSs 2 to 5. It was foundthat this sample can be used for low-quality projection displays eventhough birefringence was observed (Δ).

The sample 2 was made of SF1 (SCHOTT made) having opto-elastic constantof 1.8×10⁻⁸ cm²/N only for the first PBS (light-incident-side PBS) 2 andBK7 having opto-elastic constant of 2.77×10⁻⁸ cm²/N for the other PBSs.Birefringence was observed at the corners by close observation (∘).

The sample 3 was made as having opto-elastic constant for the first PBS2 smaller than the sample 2. In detail, SF4 (SCHOTT made) havingopto-elastic constant of 1.36×10⁻⁸ cm²/N was used for the first PBS 2 inthe sample 3. The other PBSs of the sample 3 were the same as those ofthe sample 2. No birefringence was observed (⊚).

Discussed next is birefringence against the 150 W-light source.

The sample 4 was made of BK7 having opto-elastic constant of 2.77×10⁻⁸cm²/N for the first to the fourth PBSs 2 to 5, like the sample 1. It wasfound that this sample is useless because much birefringence wasobserved (X).

The sample 5 was made of SF1 having opto-elastic constant of 1.8×10⁻⁸cm²/N for the first to the third PBS 2 and BK7 having opto-elasticconstant of 2.77×10⁻⁸ cm ²/N for the fourth PBS (light-emitting-sidePBS) 5. It was found that this sample can be used for low-qualityprojection displays even though birefringence was observed (Δ).

The sample 6 was made as having opto-elastic constant for the first tothe third PBSs 2 to 4 smaller than the sample 5. In detail, SF4 havingopto-elastic constant of 1.36×10⁻⁸ cm²/N was used for these PBSs. Thefourth PBS (light-emitting-side PBS) 5 of the sample 6 was the same asthe sample 5. Birefringence was observed at the corners by closeobservation (∘).

The sample 7 was made of SF4 having opto-elastic constant of 1.36×10⁻⁸cm²/N for the first PBS (light-incident-side PBS) 2, SF1 havingopto-elastic constant of 1.8×10⁻⁸ cm²/N for the second and the thirdPBSs (main PBSs) 3 and 4, and BK7 having opto-elastic constant of2.77×10⁻⁸ cm²/N for the fourth PBS (light-emitting-side PBS) 5.Birefringence was observed at the corners by close observation (∘).

The sample 8 was made of PBH (Ohara made) having further smallopto-elastic constant of 0.65×10⁻⁸ cm²/N only for the first PBS(light-incident-side PBS) 2 and SF2 having opto-elastic constant of2.62×10^(−8 cm) ²/N for the other PBSs. Birefringence was observed atthe corners by close observation (∘).

The sample 9 was made of PBH6 having opto-elastic constant of 0.65×10⁻⁸cm²/N for the first PBS (light-incident-side PBS) 2, SF1 havingopto-elastic constant of 1.8×10⁻⁸ cm²/N for the second and the thirdPBSs (main PBSS) 3 and 4, and SF2 having opto-elastic constant of2.62×10⁻⁸ cm²/N for the fourth PBS (light-emitting-side PBS) 5. Nobirefringence was observed (⊚).

Discussed further is birefringence against the 200 W-light source.

The sample 10 was made of SF1 having opto-elastic constant of 1.8×10⁻⁸cm²/N for the first to the fourth PBSs 2 to 5. It was found that thissample is useless because much birefringence was observed (X).

The sample was made of PBH6W having opto-elastic constant of 0.65×10⁻⁸cm²/N only for the first PBS (light-incident-side PBS) 2 and SF2 havingopto-elastic constant of 2. 62×10 ⁻⁸ cm²/N for the other PBSs. It wasfound that this sample can be used for low-quality projection displayseven though birefringence was observed (Δ).

The sample 12 was made of PBH6 having opto-elastic constant of 0.65×10⁻⁸cm²/N for the first PBS (light-incident-side PBS) 2, SF1 havingopto-elastic constant of 1.8×10⁻⁸ cm²/N for the second and the thirdPBSs (main PBSs) 3 and 4, and SF2 having opto-elastic constant of2.62×10⁻⁸ cm²/N for the fourth PBS (light-emitting-side PBS) 5.Birefringence was observed on the corners by close observation (∘).

The sample 13 was made of PBH6 having opto-elastic constant of 0.65×10⁻⁸cm²/N for the first to the third PBSs 2 to 4 and SF2 having opto-elasticconstant of 2.62×10⁻⁸ cm²/N for the fourth PBS (light-emitting-side PBS)5. No birefringence observed was observed (⊚).

The sample 14 was made of PBH5 (Ohara made) having the smallestopto-elastic constant of 0.03×10⁻⁸ cm²/N only for the first PBS(light-incident side PBS) 2 and PBH6W having optoelastic constant of0.65×10⁻⁸ cm²/N for the other PBSs. No birefringence observed wasobserved (⊚).

The sample 15 was made of PBH55 having opto-elastic constant of0.03×10⁻⁸ cm²/N for the first to the third PBSS 2 to 4 and SF2 havingopto-elastic constant of 2.62×10⁻⁸ cm²/N for the fourth PBS(light-emitting-side PBS) 5. No birefringence observed was observed (⊚).

Generation of birefringence varied in accordance with the opto-elasticconstant of glasses used for the PBS optical materials. Nonetheless, thefollowings were found according to the overall evaluation of theexperiments.

Firstly, it is required that glass types be selected for each PBS tomeet the requirement Ki<Km and Ko in which Ki, Km and Ko denote theopto-elastic constants for the first PBS (light-incident-side PBS) 2,the second and the third PBSs (main PBSs) 3 and 4, and the fourth PBS(light-emitting-side PBS) 5, respectively. Substantially nobirefringence will be observed with the first PBS 2 made of glass thatmeets the requirement Ki<1×10⁻⁸ cm²/N against a 200 W-classhigh-intensity light source.

It is more required that glass types be selected for each PBS to meetthe requirement Ki<Km<Ko. Substantially no birefringence will beobserved with the first to the third PBSs 2 to 4 made of glass thatmeets the requirement Ki<1×10⁻⁸ cm²/N and Km<2×10⁻⁸ cm²/N against a 200W-class high-intensity light source.

It is furthermore required that glass types be selected for each PBS tomeet the requirement Ki and Km<Ko. Substantially no birefringence willbe observed with the first to the third PBSs 2 to 4 made of glass thatmeets the requirement Xi and Km<1×10⁻⁸ cm²/N against a 200 W-classhigh-intensity light source.

The experimental results for opto-elastic constant of PBS opticalmaterials can be applied to the first to the fourth embodiments of CSRoptical systems.

Moreover, the experimental results can be applied to a CSR opticalsystem in which all of the first to the fourth PBSs separated from eachother, for effective restriction of birefringence.

[Fifth Embodiment]

FIG. 13 is a plan view illustrating the fifth embodiment of a CSRoptical system according to the present invention.

Elements in this embodiment that are the same as or analogous to theelements in the first embodiment (FIG. 3) are referenced by the samereference numbers and will not be explained in detail.

A CSR optical system 130 shown in FIG. 13 is different from thecounterpart of the first embodiment in that third wavelength-selectivepolarization converters 14 and 15 are B-phase plates for rotating theplane of polarization of B-light component by 90 degrees. Under theconditions, the reflective SLMs 62 and 63 in the first embodiment (FIG.3) are replaced with each other in the fifth embodiment, as shown inFIG. 13.

R-light component (dot line in FIG. 13) will not be affected by thethird wavelength-selective polarization converters 14 and 15 becausethey are B-phase plates. The S-polarized light of the incident R-lightcomponent is then reflected at both polarization-splitting planes 21 and41 of first and third PBSs 2 and 4, respectively. The reflectedS-polarized light is incident to a reflective SLM 62 for R-lightcomponent and converted into P-polarized light. The convertedP-polarized light passes through both polarization-splitting planes 41and 51 of third and fourth PBSs 4 and 5, respectively, and is emittedfrom a G-phase plate 7.

On the contrary, the S-polarized light of the B-light component(two-dashed line) is reflected at the polarization-splitting plane 21 ofthe first PBS 2. The reflected S-polarized light is converted into aP-polarized light by the third wavelength-selective polarizationconverter 14 (called B-phase plate hereinafter). The P-polarized lightpasses through the polarization-splitting plane 41 of the third PBS 4and is incident to a reflective SLM 63 for B-light component.

The S-polarized light of the B-light component modulated by the SLM 63is reflected at the polarization-splitting plane 41 of the third PBS 4and converted into P-polarized light by the B-phase plate 15. TheP-polarized light of the B-light component passes through thepolarization-splitting plane 51 of the fourth PBS and is emitted fromthe G-phase plate 7.

The G-light component behaves like the counterpart in the firstembodiment (FIG. 3), and hence is not disclosed for brevity.

The incident three light components for R-, G- and B-color primarycolors are all converted into P-polarized lights at their polarizationplane, and emitted from the CSR optical system 130.

Also in the fifth embodiment, when white light is incident to the CSRoptical system 130 via the G-phase plate 6, it is separated into the R-,G- and B-light components. The separated light components are incidentto the corresponding reflective SLMs 61, 62 and 63 for modulation with avideo signal. The modulated light components are recombined and emittedfrom the G-phase plate 7.

Moreover, all structural modifications to the foregoing embodiments andalso opt-elastic constant requirements discussed above can be applied tothe fifth embodiment, for effective restriction of birefringence.

All embodiments disclosed above have the G-phase plates 6 and 7 at thelight-incident side of the first PBS 2 and the light-emitting side ofthe fourth PBS 5, respectively. However, the other wave-selectivepolarization converters can be placed there instead of the G-phaseplates 6 and 7, with one requirement that each wave-selectivepolarization converter to be placed between two PBSs be selected inaccordance with the positional relativity with the PBSs.

[Six Embodiment]

FIG. 14 is a plan view illustrating a projection display as the sixthembodiment of the present invention.

A projection display 400 is equipped with a CSR optical system 150, alight source 430 and a first polarizing plate 440 both provided at thelight-incident side of the CSR optical system 150, and a secondpolarizing plate 450 and a projection lens 460 both provided at thelight-emitting side of the CSR optical system 150. The projectiondisplay 400 is further equipped with reflective SLMs 61, 62 and 63placed on light-passing planes 3 c, 4 a and 4 b of second and third PBSs3 and 4, respectively.

The optical elements of the CSR optical system 150 that are the same asor analogous to the elements in the first embodiment (FIG. 3) arereferenced by the same reference numbers and will not be explained indetail.

The difference between the CSR optical system 150 and the counterpart inthe first embodiment is that the former has a light blockage 420 at thecenter of the CSR optical system 150 surrounded by the first to fourthPBSs 2, 3, 4 and 5. The light blockage 420 is made of a heat-resistantresin capable of light blocking such as polyimide, polyethylene, acrylicand rubber.

In operation, white light as un-polarized light emitted from the lightsource 430 is incident to the first polarizing plate 440. A specificlinearly-polarized light component of the white light only is allowed topass the polarizing plate 440. The linearly-polarized light component isthen incident to the CSR optical system 150 via a firstwavelength-selective polarization converter 6 (G-phase plate).

The linearly-polarized light component is separated into R-, G- andB-light components and recombined as all of the three light componentsare converted into another specific linearly-polarized light componentby the CSR optical system 150, like disclosed in the first embodiment.

The other specific linearly-polarized light component only is allowed topass the second polarizing plate 450 and projected onto a screen (notshown) via the projection lens 460.

During this operation, unnecessary light components L (solid line inFIG. 14) leaked from the first PBS 2 (light-incident-side PBS) areblocked by the light blockage 420 at the intersection ofpolarization-splitting planes 21, 31, 41 and 51 and surrounded by thefirst to fourth PBSs 2, 3, 4 and 5.

The unnecessary light components L will thus not be projected onto thescreen from the fourth PBS 5 (light-emitting-side PBS) via theprojection lens 460, which may otherwise occur as indicated by a dotline in FIG. 14.

As disclosed, the light blockage 420 serves to prevent generation ofbright portions on screen due to projection of unnecessary lightcomponents, thus enhancing image quality, which may otherwise be loweredas discussed for the known projection display.

[Seventh Embodiment]

FIG. 15 is a plan view illustrating a projection display as the seventhembodiment of the present invention.

The elements in this that are the same as or analogous to the elementsin the first and the sixth embodiments (FIGS. 3 and 14) are referencedby the same reference numbers and will not be explained in detail.

The difference between a projection display 500 in the seventhembodiment and the counterpart in the sixth embodiment is that theformer has first light blockages 520 at corners 22 e and 53 e of firstand fourth PBSs 2A and 5A, respectively, for which corner edges havebeen cut off to be flat.

Another difference is that the projection display 500 further has asecond light blockage 530 provided on a light-passing plane 5 c of thefourth PBS 5A but close to the second PBS 3. The second light blockage530 is also used for blocking unnecessary light components. However, thefirst light blockages 520 only are essential for this purpose inpractical use.

Like the sixth embodiment, unnecessary light components L (solid line inFIG. 15) leaked from the first PBS 2A (light-incident-side PBS) areblocked by the light blockages 520 and 530. The unnecessary lightcomponents L will thus not be projected onto a screen (not shown) from afourth PBS 5A (light-emitting-side PBS) via a projection lens 460, whichmay otherwise occur as indicated by a dot line in FIG. 15.

As disclosed, also in the seventh embodiment, the light blockages 520and 530 serve to prevent generation of bright portions on screen due toprojection of unnecessary light components, thus enhancing imagequality, which may otherwise be lowered as discussed for the knownprojection display.

[Eighth Embodiment]

FIG. 16 is a plan view illustrating a projection display as the eighthembodiment of the present invention.

The elements in this embodiment that are the same as or analogous to theelements in the sixth embodiment (FIG. 14) are referenced by the samereference numbers and will not be explained in detail.

The difference between a projection display 600 in the eighth embodimentand the counterpart in the sixth embodiment is that the former isequipped with a CSR optical system 170 having second and third PBSs 3Aand 4A smaller than first and fourth PBSs 2 and 5, in accordance withthe size of reflective SLMs 61A, 62A and 63A.

One of the features of the projection display 600 lies in short opticallength. In detail, an optical length from a light source 430 to each ofthe reflective SLMs 61A, 62A and 63A is almost equal to that from eachSLM to a projection lens 460.

Another feature of the projection display 600 lies in two pairs ofoptical couplers 640 and 650 (made of transparent glass, for example)for the shortened optical length and protection of surface reflection onthe first to the fourth PBSs 2, 3A, 4A and 5. The optical couplers 640are placed in gaps between the first and the second PBSs 2 and 3A andalso the second and the fourth PBSs 3A and 5. The optical couplers 650are placed in gaps between the first and the third PBSs 2 and 4A andalso the third and the fourth PBS 4A and 5.

Wavelength-selective polarization converters 8A and 9A also placed inthe gaps between the first and the third PBSs 2 and 4A and the third andthe fourth PBSs 4A and 5, respectively, are made small in accordancewith the size of the third PBS 4A, though not limited to this size.

Like the sixth embodiment, light blockages 670 are provided at theintersection of polarization-splitting planes 21, 31A, 41A and 51 andsurrounded by the first to fourth PBSS 2, 3A, 4A and 5. The lightblockages 670 are made of a heat-resistant resin capable of lightblocking such as polyimide, polyethylene, acrylic and rubber. The lightblockages 670 can be reformed in shape other than that shown in FIG. 16.

Like the sixth embodiment, unnecessary light components L (solid line inFIG. 16) leaked from the first PBS 2 (light-incident-side PBS) areblocked by the light blockages 670. The unnecessary light components Lwill thus not be projected onto a screen (not shown) from the fourth PBS5 (light-emitting-side PBS) via a projection lens 460, which mayotherwise occur as indicated by a dot line in FIG. 16.

As disclosed, also in the eighth embodiment, the light blockages 670serve to prevent generation of bright portions on screen due toprojection of unnecessary light components, thus enhancing imagequality, which may otherwise be lowered as discussed for the knownprojection display.

Moreover, in the eighth embodiment, the second and the third PBSs 3A and4A are made smaller than the first and the fourth PBSs 2 and 5, inaccordance with the size of the reflective SLMs 61A, 62A and 63A, thusreducing costs of the projection display 600 including the CSR opticalsystem 170.

[Ninth Embodiment]

FIG. 17 is a plan view illustrating a projection display as the ninthembodiment of the present invention.

The elements in this embodiment that are the same as or analogous to theelements in the sixth to the eighth embodiments (FIGS. 14, 15 and 16)are referenced by the same reference numbers and will not be explainedin detail.

A projection display 700 in the ninth embodiment is equipped with a CSRoptical system 180 having second and third PBSs 3A and 4A smaller thanfirst and fourth PBSS 2A and 5A, in accordance with the size ofreflective SLMs 61A, 62A and 63A, like the eighth embodiment.

Also like the eighth embodiment, an optical length from a light source430 to each of the reflective SLMs 61A, 62A and 63A is almost equal tothat from each SLM to a projection lens 460.

Moreover, like the eighth embodiment, two pairs of optical couplers 640and 650 (made of transparent glass, for example) are provided for theshortened optical length and protection of surface reflection on thefirst to the fourth PBSs 2A, 3A, 4A and 5A. The optical couplers 640 areplaced in gaps between the first and the second PBSs 2A and 3A and alsothe second and the fourth PBSs 3A and 5A. The optical couplers 650 areplaced in gaps between the first and the third PBSs 2A and 4A and alsothe third and the fourth PBS 4A and 5A.

Wavelength-selective polarization converters 8A and 9A also placed inthe gaps between the first and the third PBSs 2A and 4A and the thirdand the fourth PBSs 4A and 5A, respectively, are made small inaccordance with the size of the third PBS 4A, though not limited to thissize, like the eighth embodiment.

Like the seventh embodiment, the projection display 700 has first lightblockages 520 at corners 22 e and 53 e of the first and the fourth PBSs2A and 5A, respectively, for which corner edges have been cut off to beflat.

The projection display 700 further has a second light blockages 730provided on the inner side faces of the optical couplers 640. The secondlight blockages 730 are also used for blocking unnecessary lightcomponents, though not essential. They may be provided only if the firstlight blockages 520 are not enough for this purpose.

Another plate- or block-like light blockage may be provided at theintersection of polarization-splitting planes 221, 31A, 41A and 231 andsurrounded by the first to fourth PBSs 2A, 3A, 4A and 5A.

During operation, unnecessary light components L (solid line in FIG. 17)leaked from the first PBS 2A (light-incident-side PBS) are blocked bythe first and the second light blockages 520 and 730.

The unnecessary light components L will thus not be projected onto ascreen (not shown) from the fourth PBS 5A (light-emitting-side PBS) viaa projection lens 460, which may otherwise occur as indicated by a dotline in FIG. 17.

As disclosed, the light blockages 520 and 730 serve to preventgeneration of bright portions on screen due to projection of unnecessarylight components, thus enhancing image quality, which may otherwise belowered as discussed for the known projection display.

Moreover, in the ninth embodiment, the second and the third PBSs 3A and4A are made smaller than the first and the fourth PBSs 2A and 5A, inaccordance with the size of reflective SLMs 61A, 62A and 63A, thusreducing costs of the projection display 700 including the CSR opticalsystem 180.

As disclosed above, a color-separating and -recombining optical systemaccording to the present invention has an optical arrangement in that atleast the first polarization beam splitter located at the light-incidentside, is separated from other optical elements, at least at its one ofinner-side light-passing planes. Thermal stress applied to the firstpolarization beam splitter that tends to be heated due to three-primarycolor components passing therethrough is released from the separatedsections.

The present invention thus achieves effective restriction ofbirefringence. The present invention also provides a highly reliablecolor-separating and -recombining optical system that is hardly peeledoff at joint sections of optical elements. This is because of release ofthermal stress due to temperature change in various circumstances fromthe first polarization beam splitter.

Moreover, at least the first polarization beam splitter is made of glasshaving a small opto-elastic constant. The present invention thusachieves further effective restriction of birefringence.

Furthermore, the present invention provides a color-separating and-recombining optical system having a light blockage at thecharacter-“X”-like intersection of polarization-splitting planessurrounded by the first to fourth polarization beam splitters.

The light blockage serves to prevent unnecessary light componentsgenerated in the first polarization beam splitter located at thelight-incident side from being projected onto a screen.

The present invention thus achieves high image quality with thecolor-separating and -recombining optical system having the lightblockage used for projection displays.

What is claimed is:
 1. A color-separating and -recombining opticalsystem comprising: cubic- or square column-like first to fourthpolarization beam splitters having polarization-splitting planesintersecting each other like a character-“X”; and wavelength-selectivepolarizing converters each for rotating the plane of polarization of aspecific-color light component by 90 degrees, one of the convertersbeing placed at a light-incident side of the first splitter, another ofthe converters being placed at a light-emitting side of the fourthsplitter, the first and the fourth splitters being provided at alight-incident side and a light-emitting side, respectively, of theoptical system, the first and the fourth splitters being arranged asdiagonally opposing each other, and the remaining converters beingplaced between at least two inner facing planes of the first to thefourth splitters, wherein at least the remaining converters and three ofthe first to the fourth splitters are joined to form an optical jointcomponent with a gap located between the remaining one splitter and theoptical joint component.
 2. The color-separating and -recombiningoptical system according to claim 1, wherein opto-elastic constants forthe first to the fourth polarization beam splitters have a relationshipKi<Km and Ko in which Ki, Km and Ko denote the opto-elastic constantsfor the first splitter, the second and the third splitters and thefourth splitter, respectively.
 3. The color-separating and -recombiningoptical system according to claim 1, wherein opto-elastic constants forthe first to the fourth polarization beam splitters have a relationshipKi and Km<Ko in which Ki, Km and Ko denote the opto-elastic constantsfor the first splitter, the second and the third splitters and thefourth splitter, respectively.
 4. The color-separating and -recombiningoptical system according to claim 1, wherein opto-elastic constants forthe first to the fourth polarization beam splitters have a relationshipKi<Km<Ko in which Ki, Km and Ko denote the opto-elastic constants forthe first splitter, the second and the third splitters and the fourthsplitter, respectively.
 5. A color-separating and -recombining opticalsystem comprising: cubic- or square column-like first to fourthpolarization beam splitters having polarization-splitting planesintersecting each other like a character-“X”; and wavelength-selectivepolarizing converters each for rotating the plane of polarization of aspecific-color light component by 90 degrees, one of the convertersbeing placed at a light-incident side of the first splitter, another ofthe converters being placed at a light-emitting side of the fourthsplitter, the first and the fourth splitters being provided at alight-incident side and a light-emitting side, respectively, of theoptical system, the first and the fourth splitters being arranged asdiagonally opposing each other, and the remaining converters beingplaced between at least two inner facing planes of the first to thefourth splitters, wherein opto-elastic constants for the first to thefourth splitters have a relationship Ki<Km and Ko, Ki and Km<Ko orKi<Km<Ko in which Ki, Km and Ko denote the opto-elastic constants forthe first splitter, the second and the third splitters and the fourthsplitter, respectively.
 6. A projection display comprising: a lightsource for emitting unlinearly-polarized light; a first polarizer toallow only a first specific-linearly-polarized light component of theunlinearly-polarized light to pass therethrough; a color-separating and-recombining optical system including cubic- or square column-like firstto fourth polarization beam splitters having polarization-splittingplanes intersecting each other like a character-“X”, the first splitterbeing provided as facing the first polarizer, and wavelength-selectivepolarizing converters each for rotating the plane of polarization of aspecific-color light component by 90 degrees, one of the convertersbeing placed at a light-incident side of the first splitter, another ofthe converters being placed at a light-emitting side of the fourthsplitter, the first and the fourth splitters being provided at alight-incident side and a light-emitting side, respectively, of theoptical system, the first and the fourth splitters being arranged asdiagonally opposing each other, and the remaining converters beingplaced between at least two inner facing planes of the first to thefourth splitters, wherein at least the remaining converters and three ofthe first to the fourth splitters are joined to form an optical jointcomponent with a gap located between the remaining one splitter and theoptical joint component; reflective spatial light modulators for lightmodulation in accordance with a video signal, provided outside theoptical system, as facing each light-passing plane of the second and thethird splitters, a second polarizer provided as facing a light-emittingside plane of the fourth splitter, to allow only a secondspecific-linearly-polarized light component emitted from thelight-emitting side plane of the fourth splitter to pass therethrough;and a projection lens provided as facing the second polarizer, toreceive the second specific-linearly-polarized light component for imageprojection.
 7. A projection display comprising: a light source foremitting unlinearly-polarized light; a first polarizer to allow only afirst specific-linearly-polarized light component of theunlinearly-polarized light to pass therethrough; a color-separating and-recombining optical system including cubic- or square column-like firstto fourth polarization beam splitters having polarization-splittingplanes intersecting each other like a character-“X”, the first splitterbeing provided as facing the first polarizer, and wavelength-selectivepolarizing converters each for rotating the plane of polarization of aspecific-color light component by 90 degrees, one of the convertersbeing placed at a light-incident side of the first splitter, another ofthe converters being placed at a light-emitting side of the fourthsplitter, the first and the fourth splitters being provided at alight-incident side and a light-emitting side, respectively, of theoptical system, the first and the fourth splitters being arranged asdiagonally opposing each other, and the remaining converters beingplaced between at least two inner facing planes of the first to thefourth splitters, wherein opto-elastic constants for the first to thefourth splitters have a relationship Ki<Km and Ko, Ki and Km<Ko or Ki<Km<Ko in which Ki, Km and Ko denote the opto-elastic constants for thefirst splitter, the second and the third splitters and the fourthsplitter, respectively; reflective spatial light modulators for lightmodulation in accordance with a video signal, provided outside theoptical system, as facing each light-passing plane of the second and thethird splitters, a second polarizer provided as facing a light-emittingside plane of the fourth splitter, to allow only a secondspecific-linearly-polarized light component emitted from thelight-emitting side plane of the fourth splitter to pass therethrough;and a projection lens provided as facing the second polarizer, toreceive the second specific-linearly-polarized light component for imageprojection.